1. Technical Field of the Invention
This invention relates to an inhalation apparatus which may be used in the measurement of the volume of a patient's blood, or which may be used as a closed anaesthetic system.
2. Description of the Prior Art
It is often necessary, particularly in the case of critically ill patients in an intensive care unit, to measure the patient's blood volume in order to assess the need for a blood transfusion. One of two generally-used methods involves the measurement of red-cell volume and subsequent calculation of total blood volume from the haematocrit, the other involves the measurement of plasma volume and again subsequent calculation of total blood volume from the haematocrit. Often both methods are used and total blood volume is then obtained by adding the measured red-cell and plasma volumes.
An established method of measuring red-cell volume involves the steps of removing a small sample of blood from the patient, labelling the red cells in the sample with radioactive tracers, and re-injecting the radio-labelled sample into the patient. After a predetermined period of time, the sample will have mixed evenly with all of the blood in the patient's body. Another sample of blood is then removed and the "dilution" of the radioactive tracers is determined, which enables the volume of blood into which the sample was re-injected to be calculated, and hence the blood volume of the patient may be determined.
The above method has been established as a sufficiently accurate method of indicating blood volume, involving relatively simple procedures and using standard equipment. However, this method is time consuming and expensive and cannot be repeated at frequent intervals: therefore, if an error occurs, it may be some time before the test can be performed again. This also means that it is not possible to monitor a patient's blood volume at frequent intervals, which may be required under certain circumstances. Another drawback of the method is that it involves radioactivity, which can be hazardous.
Another known method of measuring red-cell volume again involves the steps of removing a small sample of blood from the patient, labelling the red cells in the sample, and re-injecting the labelled sample into the patient. However, in this case, the red cells are labelled with a small quantity of carbon monoxide (CO) which is injected into the sample of blood. Carbon monoxide binds to haemoglobin in red blood cells to form carboxyhaemoglobin. Thus, once the re-injected sample has been allowed sufficient time to mix evenly with the blood in the patient's body, the "dilution" of the carboxyhaemoglobin in the patient's blood, and subsequently the blood volume, may be determined.
Although the above-described method eliminates the hazards of using radioactivity, it is still time-consuming and cannot be repeated at frequent intervals: it moreover involves the risk of infection to the medical staff handling the blood samples at the bedside. A more convenient method of measuring blood volume has been proposed wherein carbon monoxide saturation of a patient's haemoglobin or carboxyhaemoglobin is first measured and the patient then inhales a small known quantity of carbon monoxide which crosses to the red blood cells through the lungs. The carboxyhaemoglobin is again measured and the total quantity of haemoglobin may thereby be calculated which, in turn, allows calculation of the patient's blood volume. Patients in intensive care units are usually unable to breathe unaided and are thus ventilated artificially by means of a tube passed into the trachea (endotracheal tube). It is therefore necessary to provide apparatus for ventilating the patient artificially whilst administering the required dose of carbon monoxide. Such an apparatus is shown in FIG. 1 of the drawings and comprises an elongate pipe 100 connected at one end via a connector 102 to the endotracheal tube, through which the patient is ventilated. A carbon dioxide (CO.sub.2) absorber 104 is provided in the pipe 100, downstream of the mouthpiece 102, and a filter 106 is also provided for preventing dust from the CO.sub.2 absorber 104 entering the patient's airways. The apparatus further comprises an oxygen sensor 108, a 2-1 rebreathing bag 110, a first inlet 112 having a one-way valve (not shown) for administering oxygen, and a second inlet 114 having a one-way valve (not shown) for administering the carbon monoxide.
Initially, the rebreathing bag 110 is filled with approximately 2 liters of oxygen and the connector 102 is attached to the endotracheal tube. The rebreathing bag is manually squeezed and the oxygen therein is forced through the pipe 100 and out through the connector 102 to the endotracheal tube to inflate the patient's lungs. Air is automatically exhaled from the lungs by the passive recoil of the chest of the patient and enters the pipe 100 from the endotracheal tube via the connector 102. Carbon dioxide in the exhaled air is absorbed by the CO.sub.2 absorber 104 so that substantially pure oxygen re-enters the rebreathing bag 110, thereby inflating it once again. The procedure described above is repeated in order to ventilate the patient and the oxygen sensor 108 ensures that the patient is receiving sufficient oxygen. As the patient is ventilated, some of the oxygen passed into the patient's lungs is used up thereby: the quantity of oxygen within the rebreathing bag 110 therefore diminishes over a period of time. Human lungs use approximately 250 ml of oxygen per minute. In order to top up the bag 110, oxygen is manually introduced into it via the one-way valve in the inlet 112 at approximately 250 ml per minute. For the purpose of measuring blood volume, a small known quantity of carbon monoxide is introduced into the pipe 100 via the one-way valve in the inlet 114. When the rebreathing bag 110 is squeezed, the carbon monoxide is passed into the patient's lungs along with the oxygen. When the patient exhales, any carbon monoxide which has not crossed the lungs is forced back into the apparatus via the connector 102 together with the rest of the exhaled air. However, carbon monoxide is not absorbed by the CO.sub.2 absorber: therefore, when the bag 110 is squeezed again, any carbon monoxide which has not crossed the patient's lungs is `rebreathed` together with the oxygen. This procedure is repeated until all of the carbon monoxide has been absorbed by the patient's lungs: this usually takes about 15 minutes.
A number of problems occur in use of apparatus of FIG. 1. Firstly, it is inconvenient to have to repeatedly squeeze the rebreathing bag for as long as 15 minutes to manually ventilate the patient. Secondly, it is necessary to manually top-up the oxygen in the rebreathing: this may lead to errors and thus insufficient oxygen reaching the patient, which is clearly dangerous. Thirdly, over-zealous inflation of the patient's lungs may cause air and carbon monoxide to escape past the cuff of the endotracheal tube into the trachea, which would cause the results of the test to be inaccurate. Further, a single dose of carbon monoxide is administered through the one-way valve in inlet 114 by means of a syringe: thus the valve must allow removal of the syringe without permitting any carbon monoxide to escape; if a leak occurs in the one-way valve, carbon monoxide will be allowed to escape, leading to inaccurate results.
We have now devised an inhalation apparatus which may be used for blood volume measurement, and enables the problems outlined above to be overcome.
In the case of anaesthetic systems, it is usual for a predetermined mixture of anaesthetic vapour and oxygen to be supplied from an anaesthetic supply unit to a "circle" which includes a carbon dioxide absorber, one-way valves and a connector to the tracheal tube. A ventilator is connected into the circle and alternately supplies oxygen to the circle, causing inflation of the lungs, and then vents to atmosphere (for exhaled gas to pass into the atmosphere). The patent does not absorb all of the anaesthetic vapour on each inspiration, and accordingly some of the anaesthetic vapour is vented to atmosphere upon exhalation. Accordingly, some of the anaesthetic substance (which is expensive) is wasted, and furthermore pollutes the atmosphere.
We have now devised an inhalation apparatus which may be used as an anaesthetic system to overcome the foregoing problems.